The purpose of this study is to determine the effects of text lengths on students' vocabulary and to reveal students' opinions about their vocabulary development considering text lengths. This study used exploratory sequential mixed method pattern, which combines quantitative and qualitative data gathering techniques and combines research results. The study group constitutes 7th grade students studying at a secondary school in Turkey. Within the scope of the research, "Vocabulary Success Test (VST)" was used as quantitative data gathering tool, and "Structured Feedback Form (SFF)" was used as qualitative data tool. In the quantitative analysis of the research data, considering the structure of the data, t-tests were used in dependent and independent groups using SPSS-PASW Statistics 18. Descriptive analysis technique was used in the qualitative analysis of the research data. According to the results of this research, a statistically significant difference was found between the success scores of the short texts pretests and post-tests of the experimental group; it is concluded that there is no significant difference in long texts. The students in the research group stated that the short and concise texts in the course books increased their motivation towards the course.